Very likely the existing database queries that are touching tables that would be now in different cluster - which I understand wouldn't work anymore
Those queries should probably be changed to get the same data in a way that work in the new setting

Example queries

1. Getting pages (with usage details or without) that make use of a particular Wikidata items, e.g.

https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/5c12f7e7ea876d4577da4a5726de85dda1e169ba/client/includes/Specials/SpecialEntityUsage.php#L138-L154
https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/5c12f7e7ea876d4577da4a5726de85dda1e169ba/client/includes/Api/ApiListEntityUsage.php#L171-L200

A fairly naive way of rewriting those queries would be to first fetch page IDs (and aspects if relevant) from wbc_entity_usage and then query page title and names from page using a list of IDs (check for sensible size of the list, might need to batch it)

Proof of concept is done - Next check-in/status update Friday 24.07.

Change #1314026 had a related patch set uploaded (by Seanleong-wmde; author: Seanleong-wmde):

[mediawiki/extensions/Wikibase@master] Add virtual-wikibase-entityusage virtual domain

https://gerrit.wikimedia.org/r/1314026

Hi @Ladsgroup! I have some questions regarding the difference between getting the connection to the terms table and the entity usage table.

1. What is the reason the connection to the terms table in RepoDomainTermsDb.php uses RepoDomainDb->connections() directly, whereas the connection to the entity usage table in "DirectSqlStore.php" uses clientDomainDb->sessionConsistentConnections()? Specifically, the reason for the difference between connections() and sessionConsistentConnections() in both cases?

2. If it's needed, perhaps a similar logic to the sessionConsistentConnectionManager.php can be implemented in the LBFactory.php when returning a primary database to the X1 cluster?

3. As I am also referring to this patch, for example in the VirtualTermsDomainDb.php, does it make sense that instead of passing and accessing an ILBFactory directly, a ClientDomainDbFactory->newVirtualDomainDb( $domain ): VirtualDomainDb is created instead? This way, we can inject the client VirtualDomainDb directly and also have access to sessionConsistentConnectionManager.

4. I think you have heard about this already, but is there a way to test out the implementation with the virtual cluster rather than creating a table locally and going through the use cases manually?

Thank you!

Update: questions answered from communication between Amir and Jakob, thanks!